This research study will look at 24-hour (circadian) rhythms that are "free-running." That is, they are not synthesized to the 24-hour day. Circadian rhythms are generated by an area of the brain, sometimes called the "circadian clock." These rhythms include body temperature, sleepiness and the secretion of certain hormones. The circadian clock is ordinarily synchronized to the light-dark cycle; this is probably the reason that free-running (non 24-hour) circadian rhythms occur commonly in people who are totally blind. Our recent studies have shown melatonin's ability to entrain free-running rhythms in blind people to a 24 hour day. The goals of this study are: A) to examine what dose is necessary to prevent circadian rhythm from free-running once it has been entrained with melatonin treatment; and B) to look at the relationship of each individual circadian rhythm to the dose of melatonin necessary to entrain to a 24 hour day.